1. Field of the Invention
This invention relates to the construction of hydrophone acoustic arrays of the type which are towed behind vessels for seismic exploration and ship or submarine detection.
2. Discussion of the Background
Conventional towed acoustic arrays have consisted of a number of acoustic sensors, linked together with power and data cables and strength members, surrounded by a flexible enclosure and filled with a low density fluid to achieve neutral buoyancy. In recent years, military applications especially have provided an impetus for a progressive and continual improvement in performance, together with a quest for a reduction in array diameter. New materials, such as polyurethane thermoplastics, and strength members formed of aramid fibres, such as those sold under the Registered Trade Mark "KEVLAR", have enabled the implementation of performance improving techniques, such as the co-extrusion of the strength members within the polyurethane jacket forming the outer wall of the array. Such techniques are well established and are known, for example, from French Patent No. 80,27511, awarded to R L Gason Bon in 1980. Furthermore, with ministurization of the sensors and their associated electronics has come a reduction in the diameter of the array.
However, a number of limitations in this type of array still exist. As shown in FIG. 1, a prior art array 1 of this type basically consists of a number of acoustic modules 2, each containing a number of acoustic sensors 3 spaced at intervals along the length of the module 2. Each sensor 3 (which is normally comprised of a number of separate elements) is connected to data-carrying cables 4 running along the length of the array 1. Strength members 5 are also provided along the length of the array to withstand the towing strain when the array is towed. The strength members 5 and the data cables 4 are separated by a spacer 6 and the whole assembly is encased in a polyurethane hose wall 7. The interior spaces 8 in the array modules are filled with a low density fluid so as to counteract the weight of the components and thus provide an array with neutral buoyancy overall.
The problems with these types of arrays include the high cost of manufacture (due to the labor-intensive processes involved), minimal robustness (putting constraints on array handling systems), array self-generated (due to waves travelling in the internal fluid-filled hose), and leakage of the fluid (resulting in a loss of buoyancy and hence degradation of performance).
Arrays of this configuration usually have a large minimum bending radius, below which buckling of the hose wall occurs. When this happens in storage, for example on a winch drum, the material can take on a permanent set, resulting in increased array self-generated noise upon on subsequent deployments. Array self-generated noise is also due to waves travelling in the fluid filling the array and reflecting and scattering from any internal discontinuities provided by hose wall spacers, the acoustic sensors and their interconnecting wiring. Any increase in array self-noise degrades the acoustic performance of the array. Any leakage, whether due to a failed joint or a tear in the array hosewall, will result in deflation of the array, and subsequent ingress of sea-water with associated damage of the electronics in the array.